In general, this is a bad call. The object class is pretty much the last characteristic I determine when writing a skip, and not the first. Start with the story, build the anomaly, and once you know the anomaly you need to be able to tell the story you want, then you'll be able to say what the Object Class is. Starting by saying it's Keter basically means it's more important to be Keter than it is to… do anything else.

1 1/2. i was thinking that the person being called is SCP-XXXX-1

Unimportant, really. Definitely not worth determining first. There's no need to throw around extra numbers when you don't need to.

7. was once thought of as harmless and not a threat
8. …until many personnel were found dead after being called by him. Cause of death unknown.

Why? Is it necessary to randomly kill people?

9. Hanging up = bad idea, why you may ask? Well that's never mentioned. mystery and suspense right???

Wrong. This is the wrong kind of suspense. Everyone is going to think, "Just hang up." So, when people don't, it's not mysterious, the victims are just stupid.

I'm sorry, I can't recommend the idea as written. The idea is being formed in the wrong order. It should start with why and how the calls are happening, and end with the specifics. And, more importantly, what you've got is something that just kills people at random and can't be stopped. That's not interersting. That's not engaging. There's no defense, no pattern, no reason to care. What choices can people make that matter? Furthermore, an entity that knows lots of information is utterly useless, if the only effect after the call is killing the called.

In general, this is a bad call. The object class is pretty much the last characteristic I determine when writing a skip, and not the first. Start with the story, build the anomaly, and once you know the anomaly you need to be able to tell the story you want, then you'll be able to say what the Object Class is. Starting by saying it's Keter basically means it's more important to be Keter than it is to… do anything else.

I can't applaud this paragraph enough. If you look at any one of my line-by-line reviews, you'll notice that I always hit the Object Class at the end. That's because it's derived from the article, not a defining feature of the article.

What if Foundation staff just don't answer calls from numbers pertaining to people in their personal life? I'm not entirely sure that non-Foundation family or friends would even have the numbers of a Foundation site.

Why does this thing need to kill people as the ending of the story? We've got tons of things that kill people. AND THEN THEY DIE isn't particularly scary when considering all the other stories that end that way.

Hanging up = bad idea, why you may ask? Well that's never mentioned. mystery and suspense right???

The rest of the story isn't interesting enough to merit keeping this a secret. If you want to have confusing plot holes, the rest of the story needs to be strong enough to keep the reader occupied and not questioning the unexplained too much. A reader may be more likely assume that "oooh mysterious" is just laziness on part of the author.

he also knows a large huge amount about the Foundation and all the SCPs

This is pretty cliche and usually gets interpreted by readers as a last-ditch attempt to make this thing more threatening than it would be otherwise. If you really want to include something as big as a major information leak, it needs to have an in-universe reason besides "so the Foundation will actually be worried".